1. Field of the Invention
The present invention relates to an inkjet recording apparatus and an inkjet recording method, and more particularly to an inkjet recording apparatus and an inkjet recording method by which printing is carried out with an aqueous ultraviolet-curable ink ejected from an inkjet head, onto a recording medium which is conveyed through a plurality of drums, and the recording medium is dried after printing, whereby the ink is fixed.
2. Description of the Related Art
In an inkjet recording method in the related art which uses an aqueous ultraviolet-curable ink, the water in the ink bleeds into the recording medium, such as paper, the recording medium swells and deforms due to the water being absorbed by the recording medium, and wrinkles known as “cockling” arise. After drying, the paper retains wrinkles and curl, thus leading to decline in the image quality. With the increased image resolution and increased recording speeds in inkjet printers, it is necessary to achieve efficient drying at high speed and to suppress cockling.
Furthermore, if there is moisture in the ink when ultraviolet light is irradiated, then problems arise in that ink curing does not progress efficiently and high-speed fixing cannot be achieved.
Japanese Patent Application Publication No. 2002-144528, for example, does not use an aqueous ultraviolet-curable ink in particular, but describes a drying apparatus in which, in order to suppress the occurrence of wrinkles or curl, cut sheet printing paper is held by suction through a plurality of suction holes arranged on the outer circumferential surface of a cylindrical printing paper suction roller, the printing paper is rotated while tightly held on the surface of the round cylindrical printing paper suction roller, a hot air flow drying region and a cold air flow drying region for drying the printing paper are arranged at suitable positions on the outer perimeter of the printing paper suction roller, and the printing paper is dried by passing through these drying regions.
Japanese Patent Application Publication No. 2007-144773, for example, discloses an inkjet recording apparatus which records an image by applying heat energy to a recording medium which is held on the outer circumferential surface of a drum by electrostatic attraction or suction by a holding mechanism, and then releasing the holding of the recording medium at least partially and leveling the surface of the recording medium by a pressing roller, subsequently holding the recording medium again with a holding mechanism, depositing ultraviolet-curable ink by a recording head, and curing and fixing the ink by irradiating ultraviolet light onto same.
Japanese Patent Application Publication No. 2007-245653, for example, discloses an image forming method using an aqueous ultraviolet-curable ink, in which in order to achieve high-speed fixing on a recording medium, a hot air flow is blown onto an image by an image hot air flow blowing device to evaporate off water and volatile components, whereupon the image is cured and fixed by irradiating ultraviolet light of maximum intensity by an ultraviolet light irradiation device.
However, in an inkjet recording apparatus using aqueous ultraviolet-curable ink, if fixing by irradiation of ultraviolet light is performed after a recording medium has been held by suction on a drum and dried and while the recording medium is still held by suction on the same drum, then there is a problem in that curl in a convex shape arises due to the recording surface side of the recording medium adopting the shape of the drum. Furthermore, due to the suction holes of the drum through which the recording medium is still held by suction, the corresponding portions of the recording medium become slightly depressed, and the recording medium is fixed while including these depressions, thus giving rise to problems of fixing non-uniformities, drying non-uniformities or luster non-uniformities.
Moreover, if the ink is cured by ultraviolet light after the ink has been dried while the recording medium is still held by suction on the conveyance device, there is a problem in that curing non-uniformities arise due to drying non-uniformities caused by the suction holes, as well as fixing wrinkles due to curing and contraction of the ink upon fixing between the held areas and the unheld areas. Furthermore, if fixing is carried out without the recording medium being held tightly by suction on the fixing drum when the ink is cured and fixed by irradiating ultraviolet light, then the recording medium cannot be conveyed stably, fixing non-uniformities arise, and the image quality becomes instable.
Furthermore, for example, Japanese Patent Application Publication No. 2005-022398 discloses a double-side cut sheet printing machine in which, in order to enable printing of high quality by raising the drying efficiency and performing uniform drying through moving the sheet in a stable fashion, a drying apparatus having an irradiation lamp which dries the sheet is arranged in the peripheral area of a first intermediate drum which includes a gas blowing device for blowing a preheated gas onto the back surface of the sheet in order to promote drying of the back surface of the sheet and a second intermediate drum which has a gas sucking device for holding the sheet by suction.
Moreover, for example, Japanese Patent Application Publication No. 2008-179012 discloses an inkjet recording apparatus in which, in order to rapidly dry ink on a sheet conveyed by a sheet conveyance member, ink is ejected onto a printing surface of a sheet from a plurality of nozzles of an inkjet head to perform printing, whereupon the sheet which has been separated from the sheet conveyance member is held by suction onto the outer circumferential surface of a rotating body with the printing surface of the sheet facing outward, and the ink on the sheet is dried by a heating body arranged so as to oppose the outer circumferential surface of the rotating body. However, there is no mention of a device for suppressing cockling, and therefore no description relating to the design of the suction unit.
Japanese Patent Application Publication No. 2003-211749, for example, mentions the design of a suction unit and describes an image forming apparatus in which the opening ratio is decreased toward the downstream side of the conveyance direction of the recording medium. By making the opening ratio smaller toward the downstream side, when the vicinity of the leading end portion of the recording medium is held by suction by an opening section formed on the upstream side of the conveyance direction, it is possible to suppress excessive reduction of the negative pressure in the portion which is sealed off by the recording medium. However, since a composition is adopted in which the suction surface is not moved and the recording medium is conveyed by sliding over the suction surface while being held by suction from the back surface through suction holes, then the recording medium cannot be conveyed if the suction force is made too strong and problems such as tearing or jamming of the paper occur. Moreover, with a suction force sufficient to enable conveyance by sliding over the suction surface, it is not possible to suppress or correct cockling after recording. Furthermore, in a region where the opening ratio is large, in other words, where the suction hole diameter is large, the amount of depression of the paper caused by suction becomes extremely large, thus impairing the quality of the recorded object. In particular, when an image is formed on the image area, the depression of the paper is especially marked due to the reduced stiffness of the paper caused by permeation of water. Furthermore, since the temperature during drying is liable to decline in the region of the suction holes as a result of suction, then the larger the diameter of the holes, the more likely it is that drying non-uniformities will occur, and when carrying out fixing by application of heat or pressure, or UV monomer curing, there is a concern that decline is liable to occur due to residual water in the region of the holes.
Furthermore, Japanese Patent Application Publication No. 2007-144848 describes being able to achieve stable conveyance by preventing the likelihood of floating up of the end portions of a recording medium as a result of change in the air flow during high-speed conveyance, through providing suction holes at greater number in the positions corresponding to the end portions of the paper than in the central portion. However, the suction force is liable to be insufficient in the central portion of the paper where ink droplet deposition rate is relatively high, and it is not possible to suppress cockling after recording in a sufficient manner. Furthermore, since the suction force in the end portions is strong, then there is a problem in that cockling is liable to become concentrated in the central portion of the paper.
When paper is held by suction after image formation, then there is a tendency for cockling to become concentrated mainly in the central portion of the paper. Therefore, in the central portion of the paper, a large suction force is required and the opening ratio must be set to a large value. Here, if it is sought to achieve this by increasing the diameter of the suction holes, then the amount of depression due to suction becomes large in the case of thin paper, and quality is impaired. Furthermore, in paper in which cockling progresses quickly, such a matt paper or thin paper, if the holes are arranged too densely over the whole area of the paper, then there is nowhere for existing cockling to escape to when the paper is held by suction, the cockling grows abnormally in particular locations on the paper, and wrinkles may occur.
Further, recording media of a plurality of types having various different thicknesses cannot be held by suction readily in a stable fashion without the occurrence of wrinkles or floating, etc., on a suction drying drum which dries a recording medium by holding the medium by suction on an outer circumferential surface of the drum, and furthermore, if it is sought to smoothen the recording medium by removing wrinkles and floating through bringing a smoothening member into contact with an image recording surface of the recording medium which has not yet been dried, then since the recording medium is not dry, there is a problem in that image distortion and wrinkling occur and image defects occur.